The present invention relates in general to a dual bridge amplifier for processing two input signals and providing two amplified output signals into two separate loads, comprising two signal amplifiers and one common buffer amplifier. Such an amplifier is known per se, for instance from European patent application 0.492.291.
The present invention relates particularly to an amplifier for amplifying signals in the audible frequency range, for driving a loudspeaker. Therefore, the present invention will hereafter be further explained for this field of the art. However, it is noted that the present invention is not limited to such application.
For driving a load such as a loudspeaker, an amplifier stage can be configured in several ways. In a single-ended configuration, one terminal of the loudspeaker is coupled to an output terminal of the amplifier through a capacitor for eliminating any DC signal components, while the other terminal of the loudspeaker is connected to mass. In case two loudspeakers are to be driven, for instance in case of a stereo signal, or in a car audio system comprising loudspeakers in the front and in the back, two amplifiers are necessary, each amplifier having an output terminal coupled to a first terminal of a respective loudspeaker through a respective capacitor, while the second terminals of said loudspeakers are connected to mass. In such a configuration, three output terminals are necessary for connecting two loudspeakers, one of said output terminals being a mass output terminal.
In another configuration, an amplifier stage for driving one loudspeaker comprises two amplifiers, each of said amplifiers having an output terminal connected to a respective terminal of the loudspeaker. Such configuration offers many advantages over the single ended configuration. No mass terminal is necessary, and also no capacitors are necessary for eliminating a DC signal component. Further, the loudspeaker can be driven with a relatively large power whereas each of said amplifiers is loaded only in a relatively limited extent. An amplifier stage according to such configuration will be indicated as a bridge amplifier.
For driving two loudspeakers with a bridge amplifier, each of said loudspeakers being connected in the above-described way, a total of four amplifiers would be necessary. However, it is possible to use one amplifier in common for both loudspeakers: an output terminal of a first amplifier is connected to a first terminal of one loudspeaker, an output terminal of a second amplifier is connected to a first terminal of the second loudspeaker, and an output terminal of the third amplifier is connected to the second terminal of the first loudspeaker as well as to the second terminal of the second loudspeaker. This third, common amplifier is also indicated as buffer amplifier. A general advantage of such configuration is the fact that only three output terminals are necessary for driving two loudspeakers in a bridge amplifier configuration.
An example of such dual bridge amplifier configuration is described in EP-A-0.492.291. A general objective of the present invention is to provide a dual bridge amplifier stage with improved properties with respect to the dual bridge amplifier stage as described in said publication.
A basic problem of such a configuration is the fact that the buffer amplifier has a certain non-zero output impedance. This results in a bad channel separation, as can be understood as follows. In a situation where a first one of the input signals is zero and the other input signal is non-zero, the output of the corresponding first amplifier should be zero and the output of the corresponding second amplifier should reflect the non-zero second input signal. This output of the second amplifier causes a current through the corresponding second loudspeaker, and this current causes a voltage signal at the output terminal of the buffer amplifier due to the non-zero output impedance of this buffer amplifier. As a consequence, a signal current is generated in the first loudspeaker, despite the fact that the input signal for this first loudspeaker is zero.
Further, said output impedance, which in practice is non-linear, causes a relatively large distortion component in the overall output signal.
It is a specific object of the present invention to improve the channel separation of a dual bridge amplifier stage.
According to an important aspect of the present invention, the channel separation of a dual bridge amplifier stage is improved by utilizing a feedback coupling over the loudspeaker.